anacron logrotate failure on prado: /var/log/syslog.1.xz: File exists

every morning, since 4-5 days now:

Date: Thu, 29 Oct 2015 06:25:22 +0100 (CET)
From: Cron Daemon <root@prado.softwareheritage.org>
To: root@prado.softwareheritage.org
Subject: Cron <root@prado> test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
Message-Id: <20151029052522.F2BD681E19@prado.softwareheritage.org>

/etc/cron.daily/logrotate:
error: error creating output file /var/log/syslog.1.xz: File exists
run-parts: /etc/cron.daily/logrotate exited with return code 1

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Migrated from T167 (view on Phabricator)